Since lithium ion secondary batteries have high energy density per unit weight and are rapidly chargeable, these batteries have recently been studied and developed as drive power sources of electric cars and hybrid cars. As a positive active material of those lithium ion secondary batteries, there is used lithium metal oxide (e.g., NCM (LiNiCoMnO2)) capable of reversibly absorbing and releasing lithium ions. The lithium metal oxide is generally low in electron conductivity. Thus, a positive electrode plate/sheet is produced in such a manner that a paste-like coating solution prepared by mixing the above positive active material, a conductive material (e.g., AB (acetylene black)), and a binder (e.g., PVDF (polyvinylidene fluoride)) with a solvent is applied to a metal foil, and then dried and pressed.
Furthermore, the non-aqueous electrolyte solution of the lithium ion secondary batteries includes an organic solvent which is flammable. In terms of safety enhancement with respect to thermal runway, the electrolyte solution contains an overcharge additive which generates gas during overcharge (for example, CHB (cyclohexyl benzene) and BP (biphenyl) and so on) (see Patent Document 1).